This invention relates generally to an overcenter valve, and more particularly to a valve which may be precisely operated to control the feed force for a drill.
FIGS. 1 and 2 show the prior art pertinent to the instant invention. Displacement of a cylinder 10 causes motion in drill string 14, as is well known in drilling operations. A variable displacement pump 16 supplies the fluid to operate the feed cylinder 10 via a four way valve 18 and a first conduit 20.
The four way valve 18 controls the direction of flow of the fluid which is supplied to the feed cylinder 10. A feed pressure control 22 affects the pressure at which the pump 16 acts. A reservoir 17 contains the hydraulic fluid which is used by the pump 16 in extending the feed cylinder 10.
While the first conduit 20 is in fluid communication with a first end 26 of the feed cylinder 10, an overcenter valve 24 is in fluid communication with a second end 27 of the feed cylinder 10 via a second conduit 28. The overcenter valve 24 (also known as a counterbalance or a holding valve) effects the change in feed force exerted by the feed cylinder 10 as a certain feed pressure is exerted by the pump 16. A graph for this relationship is shown in FIG. 2.
The feed pressure required to open the overcenter valve 24 can be adjusted by a set spring 25. The two pressures interacting to open the valve 24 are the pressure exerted through a second conduit 28, and the pressure exerted through a pilot line 30 which is directly connected to the first conduit 20.
When overcenter valve 24 opens, fluid in a second conduit 28 is permitted to travel through the valve. An initial gradient 46 in the FIG. 2 feed force v. feed pressure graph, occurs as the valve 24, is holding pressure against the cylinder 10 rod end 27. A shallower gradient 48 occurs when the valve 24 opens due to the influence of pressure in 30 alone, so that no hold back pressure at 28 is generated.
The steeper the gradient of the graph, the less the change in feed pressure (as controlled by the operator) being necessary to produce a similar change in the feed force. In other words, the feed force is more sensitive to changes in the feed pressure. This situation can be analogized to an inexpensive transistor radio in which a minor change in the volume control produces an excessive change in the volume of the radio.
When starting to drill a hole, a small, controllable feed force is desirable to assure the hole gets started straight. Excessive downward force when starting a hole can result in a crooked hole and/or severe strain in the drill string 14 attached to the feed cylinder 10. It is thus greatly preferable to be able to more precisely control the feed force by a change in the feed pressure than occurs in the prior systems.
The forgoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.